The problem of a secure electronic time stamp occurs everywhere in applications where it is necessary to prove the time of creation of data or documents and/or the time of receipt of such electronic data. Especially due to the electronic distribution of mail, for example, in the form of e-mails, voice mails, faxes, etc., a secure time stamp becomes indispensable, especially since digital signatures require tamper-proof time information.
In principle, it is conceivable that a time signal intended for electronic time-stamping of data may be derived, for example, from a legally and/or officially recognized time signal transmitter. In Germany, such a time signal is broadcast, for example, by the DCF-77 longwave transmitter in Braunschweig. This time signal is, in fact, highly accurate, but not tamper-proof, resulting in the possibility of tampering with the transmitted time, which may lead to differences between the real time and the time specified in an electronic document. Therefore, misuse is possible in all cases where time stamps have to be paid attention to from an economic and legal point of view,
A method for transmitting official time information is already known from DE 198 45 198, where time information is fed into the networks of a mobile network operator. According to this known method, the time information is encrypted by the mobile network operator and then transmitted over the mobile network to a terminal device user, who decrypts the time information in the terminal device. In this process, secure transmission, in each case based on the network operator's technology, is provided only for the period of transmission from the mobile network operator to the terminal device user. However, tamper-proofness cannot be guaranteed for the periods until the time information is received by the mobile network operator, as a result of which misuse is possible here, especially because of the open switching centers in communications networks.